As compared to conventional 10 Gb/s wavelength division multiplexed (WDM) systems, high speed long haul WDM systems, such as 40 Gb/s and 100 Gb/s WDM systems, have approximately 16 times tighter tolerances on chromatic dispersion (CD), 4 times tighter tolerances on polarization mode dispersion (PMD), and 6 dB higher optical signal-to-noise ratio (OSNR) requirements in order to be effective. In addition, such high speed long haul WDM systems are more sensitive to nonlinearities. Thus, performance must be optimized at each and every link—every dB counts. Because margins are so tight, adaptive systems are necessary—continually compensating for changing operating conditions.
CD occurs because different colored pulses of light, with different wavelengths, travel at different speeds through optical fiber, even within the same mode. CD is the sum of material dispersion and waveguide dispersion. Material dispersion is caused by variations in the refractive index of the glass of an optical fiber as a function of frequency. Waveguide dispersion is caused by the distribution of light between the core of the optical fiber and the cladding of the optical fiber, especially with regard to single-mode fiber (SMF). CD concerns are compounded in today's high speed long haul WDM systems.
Slope mismatch dispersion is a subset of CD, and occurs in SMF because dispersion varies with wavelength. Thus, dispersion builds up, especially at the extremes of a band of wavelength channels. Slope mismatch dispersion compensation typically requires slope matching and/or tunable dispersion compensation at a receiver (Rx).
PMD results as light travels down an optical fiber in two inherent polarization modes. When the core of the optical fiber is asymmetric, the light traveling along one polarization mode travels faster or slower than the light traveling along the other polarization mode, resulting in a pulse overlapping with others, or distorting the pulse to such a degree that it is undetectable by the Rx. Again, PMD concerns are compounded in today's high speed long haul WDM systems. Further, PMD varies dynamically with temperature changes, infinitesimal asymmetries in the optical fiber core, etc., and therefore requires adaptively tunable dispersion compensation.
Regardless of the modulation format used for 40 Gb/s or 100 Gb/s WDM systems, it has been demonstrated theoretically and experimentally that the bandwidth (BW) of the Rx has an enormous impact on overall system performance, although the optimized BW varies with modulation format, for example. Currently, most Rxs include an optical filter that has been BW optimized for a given modulation format (as well as a given data rate, power level, OSNR, residual dispersion, etc.) through simulations and experiments. Typically, this optical filter is realized as a thin film, fiber Bragg grating (FBG), or planar lightwave circuit (PLC) based device. Any change in modulation format and/or any of the other characteristics of the transmission system requires a new optical filter design. In addition, manufacturing tolerances can often prevent the optical filter from demonstrating BW optimized properties. Thus, what is needed in the art is a performance optimized Rx with a BW adaptive optical filter that compensates for changes in modulation format and/or any of the other characteristics of the transmission system.